A new report presents hard evidence for the benefits of awnings in helping provide more sustainable housing.
Editor’s note: This is the second of two energy studies conducted by the University of Minnesota’s Center for Sustainable Building Research under the support of the Professional Awning Manufacturers Association (see FA May/June 2007, pg. 14.) Copies of the full reports can be obtained at: /www.awninginfo.com.
The benefits of awnings in residential buildings
Awnings have advantages that contribute to more sustainable buildings. First, awnings result in cooling energy savings by reducing direct solar gain through windows. This directly reduces the impact of global warming from greenhouse gas emissions. A second benefit is that peak electricity demand is also reduced by awnings potentially resulting in reduced mechanical equipment costs. Reduced peak demand may also result in energy cost savings in the future if residential customers are charged higher rates during peak periods. Another outcome of peak demand reduction is the overall savings to utility companies and the public from a decreased need to build new generating capacity.
Cooling energy savings and peak demand reduction
Tables 1 and 2 show the impact of awnings on reducing cooling energy and peak demand in 12 U.S. cities with different climates. The cities are listed starting with the lowest cooling energy use (Seattle) up to the highest (Phoenix). For each city, results are shown for two typical houses. The first house has windows equally distributed on all four orientations while the second house has 80 percent of the windows facing west (the case with the highest cooling energy use from heat gain). The results in Tables 1 and 2 represent the best case for savings when awnings are applied to clear double-glazed windows and operated seasonally (details appear in the full report).
Table 1 shows cooling energy savings in all cities for all orientations, while Table 2 shows peak demand savings in most cities. In all cases, the cooling energy and peak demand savings from awnings are greater in the house with predominately west-facing windows. The highest percentage savings do not necessarily produce the highest actual savings. This occurs because some of the warmer cities with lower percentage savings have greater actual cooling energy and peak demand savings than colder climate cities with higher percentage savings and lower actual savings. Surprisingly, there can be little or no peak demand savings from awnings in some hot, humid cities. This is due to climatic variations that influence whether peak demand is driven more by solar gain through windows or by factors such as temperature and humidity. It is important to remember that these results are for a 2000 sq ft house and should be interpolated for larger houses. In addition, the energy prices may rise in the future increasing the savings and shortening the payback for investing in awnings.
Tables 3 and 4 show more extensive set of impacts from awnings for two cities: a predominantly cold climate (Boston) and a predominantly hot climate (Phoenix). Window types shown are clear double glazing, high-solar-gain low-E glazing, and low-solar-gain low-E glazing. Shading conditions include: no shading, awnings deployed 12 months a year, and awnings deployed in the summer only.
Cold Climate Impacts
Table 3 shows the impact of awnings on a typical house in Boston, Massachusetts, a predominantly cold climate. The impact varies depending on the type of window glazing and whether the awnings are in place 12 months per year or only in the summer.
For a house with windows equally distributed on the four sides, Table 3 shows the annual heating and cooling energy use and the peak electricity demand for each combination of glazing and shading condition. Table 3 also shows the impact on the total cost of heating and cooling. In each case, the table shows the percent savings compared to the unshaded condition.
As shown in Table 3, the awnings reduce the cooling energy 23–24 percent compared to a completely unshaded case. The actual savings are greater with the clear glass (A) and less with the low-solar-gain low-E glass (C). Because awnings block passive solar gain in winter, heating energy increases by 6–9 percent if the awnings remain in place 12 months a year. By removing or retracting the awnings in winter while keeping them in place in the summer, the lowest total energy use is achieved.
The total cost of heating and cooling is about equal in Boston when awnings are only used in the summer, but the total cost is increased if they remain in place 12 months a year.
Table 3 also shows that awnings reduce peak electricity demand by 17–22 percent in Boston. This may contribute to the ability to downsize the mechanical cooling system. The actual reduction is greater with the clear glass (A).
Hot Climate Impacts
Table 4 shows the impact of awnings on a typical house in Phoenix, Arizona with different orientation conditions. The same window orientation, window types, and shading conditions used for Boston are applied in Phoenix.
In Phoenix, the awnings reduce the cooling energy 14–20 percent compared to a completely unshaded case. As in Boston, because awnings block passive solar gain in winter, heating energy increases if the awnings remain in place 12 months a year. Of course, the relative importance of the heating versus the cooling season impacts varies by climate. In predominantly warm climates like Phoenix, the impact of awnings on reducing passive solar gain is less of a concern.
The total cost of heating and cooling is reduced 13–18 percent in Phoenix when awnings are only used in the summer. Table 4 also shows that awnings reduce peak electricity demand by 9–12 percent in Phoenix, potentially contributing to the ability to downsize the mechanical cooling system. The actual savings are greater with the clear glass (A) and less with the low solar-gain low-E glass (C).
In comparing Tables 3 and 4, it is clear that the impacts of awnings are different depending on the building location and whether the awnings are deployed year-round or only in the summer. A very important consideration in assessing the benefits of awnings is window orientation. A house in any climate with the windows predominantly facing to the east, south, and west will have greater cooling energy use and cooling peak demand than the equal orientation case. This is particularly true with peak demand in the west orientation. Generally, this means energy and cost savings from using awnings is greater with predominantly east, south, and west orientations than when windows are equally distributed. Specific energy and cost savings multiple orientation conditions can be found in the full report.